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Source Parameters of the 1933 Long Beach Earthquake

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Source Parameters of the 1933 Long Beach Earthquake Bulletin of the Seismological Society of America, Vol. 81, No. 1, pp. 81- 98, February 1991 SOURCE PARAMETERS OF THE 1933 LONG BEACH EARTHQUAKE BY EGILL HAUKSSON AND SUSANNA GROSS ABSTRACT Regional seismographic network and teleseismic data for the 1933 (M L -- 6.3) Long Beach earthquake sequence have been analyzed, Both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the Newport-lnglewood fault. The focal mechanism had a strike of 315 °, dip of 80 o to the northeast, and rake of - 170°. Relocation of the foreshock- main shock-aftershock sequence using modern events as fixed refer- ence events, shows that the rupture initiated near the Huntington Beach-Newport Beach City boundary and extended unilaterally to the northwest to a distance of 13 to 16 km. The centroidal depth was 10 +_ 2 km. The total source duration was 5 sec, and the seismic moment was 5.102s dyne-cm, which corresponds to an energy magnitude of M w = 6.4. The source radius is estimated to have been 6.6 to 7.9 km, which corresponds to a Brune stress drop of 44 to 76 bars. Both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents. When the slip estimate from the seismic moment of 85 to 120 cm is compared with the long-term geological slip rate of 0.1 to 1.0 mm I yr along the Newport-lnglewood fault, the 1933 earthquake has a repeat time on the order of a few thousand years. INTRODUCTION The 1933 Long Beach earthquake was the second largest earthquake (after the 1971 San Fernando earthquake) to strike the Los Angeles area in the twentieth century. It caused extensive damage in the greater Los Angeles area and in particular, throughout the southern part of the Los Angeles basin (Fig. 1). Many hundreds of people were injured and 120 people died (Wood, 1933). This widespread damage was in part caused by the lack of a seismic safety element in building codes (Richter, 1958). The Richter magnitude scale had not been devised at this time, and Wood (1933) described the main shock not as a great earthquake but as "a fairly strong, moderately large local shock." Richter (1935) assigned a magnitude of about 6.2 to the main shock by comparing the recorded amplitudes at Tinemaha and Haiwee in eastern California with the amplitudes of a M L = 5.5 aftershock. Later, Rich'ter (1958) refers to the more commonly known value of M L = 6.3 of the 1933 earthquake. In the late 1920s, the Caltech seismographic network with seven stations was installed to monitor earthquake activity in southern California (Wood, 1933). The 1933 Long Beach earthquake was the first damaging large earthquake to occur within this network and was well recorded. In addition, the main shock was recorded by three strong motion accelerographs, one in Long Beach and two in downtown Los Angeles (Heck, 1933; Heck and Neumann, 1933). The earth- 81 82 E. HAUKSSON AND S. GROSS T T i i a'2 o' i i if" oo' S I i 0 /[, AREA OF SERIOUS DAMAGE TO WEAK MASONRY &MOUNT WILSON ( MODIFIED MERCALLI G A B R l ~" L M 0 U N "I- A l N INTENSITY -Ol'rr ) h,~/~ ........ OUATERNARY ALLUVIUM PASADENA .. l ...................::::: c'..., ::........ :"~ oo, ........ ........ .......... O oMO-- .,,"' ,..," .......... %. " r WHITTtE~,,,.,d:' 34"o0'" \ I~ ..... ~x ~ ''['~..,"~.OCT. "22 1941 ~"~#:.. ............ -- ,~ "4. v..%~uEs °~/~i'~"~-~ ~ ;% ' k_)G~ovE ] ', 4~ oo, ..... Y ............. *4 \ ....... .......... % \ - ,0" -- ~ HUNTINGTON'~%I~ // .......%, "~ ,Z~ . f'3~4o' uC, ~ , EACH "%'~.,~,~" # '... ., ,,:7/ ',,c "~ <" 4~'-'¢ 4/ ~ ............., , , ,OKM. *'.9 MAR lO 1933 0 * i n I0 MILES FIG. 1. Map of the Los Angeles and Orange County areas shaken by the 1933 Long Beach earthquake. Important epicenters (solid circles) and the Modified Mercalli intensity line of VIII are also shown. Dotted curve outlines the Quaternary alluvium. Solid triangles indicate the location of seismograph stations. Adopted from Richter (1958) with modifications from Barrows (1974). quake was also recorded at teleseismic distances by long-period seismographs in operation throughout the world and, in particular, in the United States and many European countries. The 1933 Long Beach earthquake has not been studied in as much detail as some other similar damaging earthquakes, such as the 1971 San Fernando earthquake. This paper reanalyzes some of these old regional and teleseismic data using modern techniques to enhance our understanding of this sequence and to confirm or negate previous findings. Newer and more advanced tech- niques make it possible to resolve more of the temporal and spatial details of the earthquake faulting and to determine or refine the existing estimates of source parameters such as focal mechanism, seismic moment, and centroidal depth for the main shock. At the time of the earthquake, epicenters were calculated by hand. Because the volume of data was large, many of the aftershocks that occurred during the first few weeks were arbitrarily assigned identical epicenters (Fig. 2; Wood, 1933; C. F. Richter, unpublished data, 1933). Because one reliable felt report was available from south Long Beach, most of the aftershocks were assigned this location (Wood, 1933). It is therefore impossible to evaluate the spatial extent of the aftershock zone or possible spatial clustering of the aftershocks from these locations. The first objective of this study has been to remedy this problem by rereading the old seismograms and relocating the aftershocks. The second major objective of this study is to determine the teleseismic focal mechanism of the main shock. The source mechanism of the 1933 earthquake SOURCE PARAMETERS OF THE 1933 LONG BEACH EARTHQUAKE 83 1933 Long Beach Sequence CIT Catalog; First 6 Days of Aftershocks LOS ~i~ MAGNITUDES Angeles / ~ o 0,0+ .\ • • • , ~ 5.0+ " . ~ 6.0+ 50' Nearly all aftershocks were 'o~ Iz_ . -" assigned this location \ \ 40' \ M=6.3 \ Huntington Beac \ \ \ \ Ne" Beach \\ 0 \ \\ x 20 KM [ I I I I I h i i i I i I I I I I I l l \\\ 33 ° ,\ 20' 10' 118 ° 50' FIG. 2. The epicenters of the main shock and first 6 days of aftershocks as published in the Caltech Earthquake Catalog (Hileman et al., 1973). Events with M >_ 5.3 are shown as stars and events with M < 5.3 are shown as open circles. Most of the events are assigned one location in southern Long Beach. was first studied by Benioff (1938), who used the elastic rebound theory to show that the source was not a point source but had a finite spatial extent. He also reinterpreted the azimuthal distribution of tombstones, as determined by Clements (1933), to show that the faulting was strike-slip. Barrows (1974) reviewed the geological and seismological aspects of the earthquake and argued that the event occurred on the Newport-Inglewood fault (NIF). In a more recent study, Woodward-Clyde (1979) determined a strike-slip focal mechanism that was not consistent with the earthquake being caused by the Newport-Inglewood fault, unless some complex effects of the three-dimensional velocity structure of the Los Angeles Basin were included. Recent studies of the seismotectonics of the Los Angeles basin (Hauksson, 1987, 1990) and compressional structures such as folds (Davis et al., 1989) suggest that thrust faulting in the Los Angeles basin is more common than previously thought. Recently, Suppe (1989) suggested that the NIF has a significant thrust faulting component. Therefore it is important to reevaluate the data from the 1933 Long Beach earthquake to investigate whether it had a significant thrust faulting component. The focal mechanism is also used to determine other source parameters, such as seismic moment and stress drop. The seismic moment estimated by Thatcher 84 E, HAUKSSON AND S. GROSS Seismograph Stotions in 1935 119 ° 118 ° 117 ° FIG. 3. The Caltech seismographic network that began operating in the late 1920s. The stations PV and LG were temporary sites were one portable station was deployed to record the 1933 aftershocks. and Hanks (1973) using a single station determination at Pasadena (PAS) and by Woodward-Clyde (1979) using teleseismic waveforms are reevaluated using the focal mechanism determined in this study. The average stress drop is determined using the seismic moment and the source radius derived from the source time function. EARTHQUAKE LOCATIONS Seismographic Network The Caltech seismographic network in operation at the time of the 1933 earthquake is shown in Figure 3. Richter (unpublished data, 1933) used data from these stations to determine the epicenters of the earthquake and some of the aftershocks. Each station was equipped with both vertical and horizontal seismometers and photographic recording. Two stations, PV and LG, were temporary sites occupied with a portable station for a few days at each site (C. F. Richter, unpublished notes, 1933). Time Corrections To relocate the aftershocks, the P and S arrival times for 94 events were reread from the photographic records. An additional 12 events, with four arrival times each, that C. F. Richter had read himself and listed in his unpublished SOURCE PARAMETERS OF THE 1933 LONG BEACH EARTHQUAKE 85 data were also included in the dataset. The P and S arrival times measured off the records were corrected to a common time base by adding time corrections read from radio records for each station. These radio records are the same size as a seismogram and have the same time ticks generated by the local clock, but they are attached to a short-wave radio that received signals in morse code. Richter determined detailed time corrections for two of the stations, Mount Wilson (MWC) and Riverside (RVR).
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